This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to disc drive servo demodulation hardware configured to generate a position error signal (PES) from various selectable combinations of operand values derived from decoded servo position data, wherein the demodulation hardware provides the PES to a servo processor to effect head positional control.
Disc drives are used as primary data storage devices in modern computer systems and networks, due to the efficient and cost-effective manner in which large amounts of computerized data can be stored and retrieved. A typical disc drive comprises one or more rigid magnetic recording discs which are mounted to a spindle motor for rotation at a constant high speed. An array of read/write transducing heads are provided to transfer data between tracks of the discs and a host computer in which the disc drive is mounted. The heads are supported by a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop, processor-based servo system.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly measured and compared to a velocity profile defining a desired velocity trajectory for the seek. Once the head has settled on the destination track, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Both track seeking and track following operations typically require generation of a position error signal (PES) which gives an indication of the intratrack position of the head with respect to the particular track over which the head is disposed. A typical PES is provided with a magnitude of zero (0) when the head is positioned over the center of the track (xe2x80x9con trackxe2x80x9d), a magnitude of xc2x10.5 at track boundaries, and is nominally linearly proportional to relative misposition distances (xe2x80x9coff trackxe2x80x9d) between the track center and track boundaries. It is desirable to have a PES that exhibits a substantially constant slope across the width of the track as the head is swept from one track boundary to the next.
The PES is generated by using the head to transduce servo position data from the associated track to provide analog servo burst signals. A demodulation circuit of the servo system conditions the analog servo burst signals, including conversion to digital form, and provides the digital servo burst signals to a servo processor which combines the burst signals using a predetermined algorithm to arrive at the PES. As mentioned above, the PES identifies the actual location of the head with respect to the center of the track. The servo processor further identifies a desired (reference) position for the head, such as over the center of the track, or a selected percentage of track width away from the center of the track. such as +10% off-track. By differencing the PES and the desired position, an actual head position error is determined. The servo processor uses this actual position error to generate and output a digital current adjustment signal to a power amplifier circuit, which in turn adjusts the current applied to an actuator motor to adjust the position of the head accordingly. Thus, during track following the servo circuit continuously attempts to drive actual position error to zero.
To provide stable operation, the transfer function relating PES to actual radial misposition should be constant and independent of distance off track in the presence of variations in head signal amplitude, linear recording bit density, head to media spacing and head skew angle. In practice, however, these and other factors can introduce nonlinearities in the PES, adversely affecting the ability of the servo system to accurately position the heads. Since at least some of the factors that introduce PES nonlinearities are head and track location dependent, such nonlinearities are not constant across all head/track location combinations, but do typically tend to affect all head/track location combinations to a greater or lesser extent.
A continuing trend in the industry is to provide successive generations of disc drives with ever higher data storage capacities at lower cost. This has led to significant annual increases in track densities (greater than 60% per year for the past several years). As the tracks become narrower and closer together, the effects of PES nonlinearities become increasingly pronounced and consume greater amounts of servo processor resources to maintain acceptable levels of on-track performance. This can be mitigated to some extent through the introduction of higher performance, higher capability servo processors, but the introduction of such processors increases component cost and is directly counter to the demand for reduced cost drives.
There is therefore a continued need for improvements in the art to achieve robust head positional control in the face of continued increases in track densities and PES nonlinearities, and it is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for positioning a head adjacent tracks on a recording surface in a disc drive.
In accordance with preferred embodiments, a servo control circuit includes a multi-mode servo position error signal (PES) demodulator circuit having an operand generator which outputs operand values from decoded servo position data transduced by a head from a disc. The demodulator circuit further includes a PES generator which generates a PES as a sequence of PES samples indicative of head position with respect to the disc surface in accordance with a selected PES mode from a population of mutually exclusive PES modes each derived from a different combination of selected operand values.
The demodulator circuit provides the PES to a programmable servo processor, which in turn outputs a corresponding sequence of current adjustment signal samples to adjust the current applied to an actuator motor in relation to the PES and a desired position. The servo processor selectably configures the PES generator for each track so that different PES modes are used for different tracks.
The tracks are preferably arranged in a plurality of concentric zones so that user data are written to each track in each zone at a constant frequency unique to each zone, and an individual PES mode is selected for each zone. Alternatively, a different PES mode can be selected for each individual track on each surface.
To select optimal PES modes, the disc drive preferably performs steps of initially defining a population of mutually exclusive PES modes each derived from a different combination of selected operand values obtained in turn from servo position data stored on the recording surface; positioning the head adjacent a selected track on the recording surface; for each of the population of PES modes, sequentially generating a sequence of PES samples from the servo position data associated with the selected track; identifying an optimal PES mode from the population of PES modes in relation to linearity across a radial width of the selected track of the respective sequences of PES samples; and, thereafter, using the optimal PES mode to generate a position error signal when the head is positioned adjacent the selected track.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.